1. Field of the Invention
The invention relates to semiconductor wafer processing. More particularly, the invention relates to a cathode assembly with a detachable electrostatic chuck for retaining a wafer in a semiconductor wafer processing system.
2. Description of the Background Art
In semiconductor wafer processing equipment, cathode assemblies are often used as substrate supports (also known as susceptors) to retain wafers within the equipment during processing. The susceptor is typically mounted to a pedestal in a semiconductor wafer processing chamber. These susceptors find use in etching, chemical vapor deposition (CVD), physical vapor deposition (PVD) and preclean applications. In many of these applications, the susceptor contains a cathode electrode that can be biased with direct current (DC) or radio frequency (RF) voltage to enhance the process being performed in the chamber. Bias voltage is typically supplied to the cathode by an external power supply through a cable and appropriate feedthroughs. In these applications, the wafer is often subject to bombardment by energetic particles from a plasma. Such energetic bombardment heats the wafer, susceptor and pedestal to high temperatures, typically as high as 175.degree. C. and sometimes as high as 500.degree. C. Since excessive heating of the wafer is often undesirable, the susceptor and pedestal contain cooling mechanisms. Typically, the susceptor contains ports and surface channels for an inert backside gas that is used as a heat transfer medium between the wafer and the susceptor. Additionally, the pedestal contains a cooling plate having channels and tubes for circulating a cooling fluid such as water. The combination of backside gas and a cooling plate within the pedestal is generally sufficient to moderate the wafer temperature.
A particular type of susceptor is an electrostatic chuck. Electrostatic chucks secure a semiconductor wafer by creating an electrostatic attractive force between the wafer and the chuck. A voltage, applied to one or more insulated electrodes in the chuck, induces opposite polarity charges in the wafer and electrodes respectively. The opposite charges pull the wafer against the chuck, thereby retaining the wafer. For example, in a bipolar ceramic chuck, the wafer rests flush against the surface of a ceramic chuck body as chucking voltages of opposite polarity are applied to two chuck electrodes that are embedded in the chuck body. Because of the semiconductive nature of the ceramic material, charges migrate through the ceramic material and accumulate proximate contact points between the wafer and the surface of the chuck body. Consequently, the wafer is primarily retained upon the chuck by the Johnsen-Rahbek effect. Such a Johnsen-Rahbek chuck is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992 and U.S. Pat. No. 5,463,526 issued Oct. 31, 1995.
An improvement in the design of susceptors is a detachable electrostatic chuck such as that shown and described in commonly assigned U.S. application Ser. No. 09/071,784 filed May 1, 1998. Such a chuck is secured to a pedestal but easily removable to facilitate repair and replacement of the chuck. Such a chuck design must incorporate a releasable electrical connection between the cathode or chucking electrodes and their respective cables.
Prior art connections made use of "banana" plugs that have a male connector with one or more resilient contact portions that fits into a cylindrical female connector. Unfortunately, repeated connection and disconnection bends the male connectors out of alignment. Consequently, the connectors apply stress to the ceramic in which they are embedded causing cracking of the ceramic. One solution to this problem, is to use spring loaded connectors. For example, commonly assigned U.S. patent application Ser. No. 09/071,784 describes a spring loaded plate that contacts the bottom of the electrostatic chuck to achieve an RF connection. Although this type of connector works well for a DC bias, the impedance of the coil spring produces a sizable voltage drop when a high frequency (e.g. 13.56 MHz) RF voltage is applied.
Also, in a plasma environment, the hardware used to secure the chuck to the pedestal is exposed to attack from energetic particles within the plasma. Furthermore, a uniform chucking force depends upon a uniform distribution of contact points between the backside of the semiconductor wafer and the chuck surface. Since the contact point distribution varies from wafer to wafer, similar wafers are not chucked in the same manner. Furthermore, wafer backside materials may vary and consequently cause differences in the chucking force across the wafer as well as from wafer to wafer. As such, the magnitude of the chucking force and its uniformity depends on wafer backside morphology and wafer backside composition.
Therefore, a need exists in the art for a cathode assembly with a detachable ceramic susceptor, suitable for use in an RF plasma environment, that reduces the chucking force dependence upon wafer backside morphology and composition.